This invention relates to an osmotic distillation process for concentrating a liquid. More specifically, it relates to an osmotic distillation process in which a nonporous membrane of a high free volume polymer composition is utilized to transfer a volatile component from a liquid to be concentrated into a strip solution.
Osmotic distillation is a type of membrane separation process in which a component of a liquid on one side of a porous or microporous membrane is transported through the membrane to another liquid on the opposite side of the membrane. It differs from other widely recognized types of membrane separation processes, such as reverse osmosis, ultrafiltration, and pervaporation. One difference is that the internal surfaces of the membrane pores are not wetted by either of the two liquids. As in some membrane separation processes, the component to be moved across the membrane in osmotic distillation is a liquid. That is, this component exists in the liquid state at standard conditions, i.e., room temperature and atmospheric pressure. The liquids on opposite sides of the membrane are solutions in which the moving liquid component is soluble or miscible. Another difference of conventional osmotic distillation from other membrane separation processes is that the transported liquid component vaporizes at the liquid-membrane interface and migrates through the membrane pores either by convection or diffusion. The transported component is said to be xe2x80x9cvolatilexe2x80x9d. The driving force for transfer of the volatile component is the difference between the vapor pressure of the volatile component over the xe2x80x9csendingxe2x80x9d liquid and the lower vapor pressure of the component over the xe2x80x9creceivingxe2x80x9d liquid. After vaporizing from the liquid state in the sending liquid, the volatile component passes through the membrane in a vapor state and condenses into the receiving liquid upon emerging from the membrane.
A significant feature of osmotic distillation is that the transfer of the volatile component does not require a substantial system pressure or temperature gradient across the membrane. Therefore, this process advantageously can be carried out at ambient temperatures and pressures. Such low temperature and low pressure process conditions render osmotic distillation ideal for increasing the concentration in an initially dilute liquid phase of a temperature and/or pressure sensitive component. These are materials which have limited stability to elevated temperatures and shear stresses. Such a component is one which would likely be adversely affected or destroyed if concentrated at elevated temperatures or pressures required by other processes. As a result of this important characteristic, osmotic distillation recently has gained much favorable attention in particular for the potential to concentrate liquid foodstuffs, cosmetics (e.g., fragrances), pharmaceutical products and thermally labile biological substances. An excellent survey of osmotic distillation technology is presented in Hogan, Paul A., A New Option: Osmotic Distillation, Chemical Engineering Progress, July, 1998, pp. 49-61, which is incorporated herein by reference in its entirety.
The concentration of beverages such as fruit and juices and alcoholic beverages is a primary application for osmotic distillation. Perhaps the most notable reasons for concentrating beverages are that the concentrates do not contain large amounts of solvent and they are stable much longer than in the dilute state. The concentrates thus can be shipped less expensively for long distances and remain fresh far longer than if diluted.
Some fruit juices, especially those of citrus fruit have oils or other components that reduce the surface tension of the juice, e.g., surfactants. These oils and other surface tension reducing components are collectively referred to herein as xe2x80x9coilsxe2x80x9d or xe2x80x9coily componentsxe2x80x9d. For example, orange juice contains a substantial amount of dissolved limonene oils. The presence of dissolved oils in a primarily aqueous juice solution can be problematic for osmotic distillation because the oily components tend to wet the membrane surface, fill the pores and reduce or altogether block desired transmission of the volatile component, thereby preventing further concentration of the starting material. Oils may also eventually penetrate the membrane and allow the fluids on either side of the membrane to mix, which is undesirable.
Fruit juices as well as other liquids can be pulpy. That is, they contain solids suspended in the liquid. As the juice concentrates, the solid concentration increases. Solids can also block substantial portions of the membrane surface so as to occlude the pores and hinder osmotic distillation to the extent that the rate of transmission of the volatile component is greatly reduced.
It would be desirable to have an osmotic distillation process for concentrating liquids that is resistant to wetting and blocking of the membrane by oily and or solid components in the process liquids. Accordingly, there is now provided according to this invention a process for concentrating a liquid feed mixture comprising the steps of
providing a feed mixture of components present in initial concentration, the mixture comprising a volatile liquid component in which mixture said volatile liquid component is soluble and over which mixture said volatile liquid component has a first vapor pressure, and a strip solution over which the volatile liquid component has a second vapor pressure different from the first vapor pressure;
providing a two sided membrane structure comprising a nonporous membrane on one side and coextensive with a microporous substrate on the second side, the nonporous membrane comprising a polymer composition having a free volume of at least about 15% and being of a composition which is permeable to the volatile liquid component and not wettable by either the feed mixture or the strip solution;
contacting the feed mixture with the nonporous membrane while contacting the strip solution with the microporous substrate;
maintaining the second vapor pressure below the first vapor pressure, thereby causing the volatile liquid component to permeate the membrane from the feed mixture to the strip solution and thus producing a concentrated liquid mixture comprising components other than the volatile liquid component present at concentration greater than the initial concentration; and
removing the concentrated liquid mixture.
There is also provided an osmotic distillation apparatus for concentrating a liquid feed mixture of components present in initial concentration, the mixture comprising a volatile liquid component which is soluble in the feed mixture and over which said volatile liquid component has a first vapor pressure, the apparatus comprising,
a two sided membrane structure comprising a nonporous membrane comprising a polymer composition having a free volume of at least about 15% and being of a composition permeable to the volatile liquid component, and a microporous substrate adjacent and coextensive with the nonporous membrane,
means for contacting the nonporous membrane with the feed mixture,
means for contacting the microporous substrate with a strip solution comprising the volatile liquid component which is present at a second vapor pressure different from the first vapor pressure; and
means for maintaining the second vapor pressure below the first vapor pressure, thereby causing the volatile liquid component to permeate the membrane from the feed mixture to the strip solution,
in which apparatus the composition of the nonporous membrane is not wettable by either the feed mixture or the strip solution.